The answer to this question is very similar to the previous question. Gluten molecules form in the presence of water and bind very strongly to each other. The more water there is the more gluten. Also, the more dough is kneaded or worked, the more bonds made between gluten molecules creating what you can think of as a net or mesh of molecules. When making cookies, you typically add baking soda. Baking soda (also known as sodium bicarbonate) reacts with acidic compounds in the dough and releases carbon dioxide. The tightly bonded gluten molecules trap this gas, which then allows for the cookies to expand while they bake. Without gluten and the mesh of bonds it makes, these gases are not trapped and the dough collapses.